Late medieval shirt with gussets in the seams at shoulder, underarm, and hem.

In sewing, a gusset is a triangular or rhomboid piece of fabric inserted into a seam to add breadth or reduce stress from tight-fitting clothing. Gussets were used at the shoulders, underarms, and hems of traditional shirts and chemises made of rectangular lengths of linen to shape the garments to the body.[1][2]

Gussets are used in manufacturing of modern tights and pantyhose to add breadth at the crotch seam. As with other synthetic underwear,[3] these gussets are often made of moisture wicking breathable fabrics such as cotton, to keep the genital area dry and ventilated.[4][5][6]

The term "don't bust a gusset" comes from this sewing term; a gusset in this context was usually a piece of fabric sewn between two others to increase mobility or increase the size of the pant waist, the latter being more common in the early 1900s.[citation needed]

Gussets are also used when making three-piece bags. In a Boye Needle Company publication,[7] it is used in a pattern for a bag as a long, wide piece which connects the front piece and back piece. By becoming the sides and bottom of the bag, the gusset opens the bag up beyond what simply attaching the front to the back would do. With reference to the dimension of the gusset, the measurements of a flat bottom bag may be quoted as LxWxG.[8]

Pillows too, are often gusseted, generally an inch or two. The side panels thicken the pillow, allowing more stuffing without bulging.